The invention relates to a device for remote measurement of the properties of the atmosphere, more particularly a device used to detect atmospheric turbulence. The device operates on the lidar principle, using the backscattering of a laser beam by the air. The device employs a laser source, in general a pulsed laser source. The propagation of the laser pulses is tracked from receiving a fraction of the radiation re-emitted either by aerosols suspended in the atmosphere or by the air molecules of the atmosphere. Of particular interest is the reception of wavelengths very close to the wavelength of the laser source. The backscattered radiation is analyzed by interferometry. Two types of diffusion are observed. A first type, usually called Mie scattering, is scattering by aerosol particles in suspension in the atmosphere. These particles move at the speed of the wind present in the observation region in such a way that the backscattered wavelength is Doppler-shifted relative to the wavelength of the laser source by an amount corresponding to the wind speed. A second type, commonly called Rayleigh scattering, is scattering by the molecules of the air. The natural movement of the molecules generates a scattering effect, the spectral width of which is broader than that of the particles suspended in the atmosphere, but the Rayleigh line always remains centred on the Mie line. On passing through an interferometer, the two lines (Mie and Rayleigh) are superimposed and it is difficult to separate them without using sophisticated equipment, which equipment is generally reserved for studies carried out in the laboratory. The aim of the invention is to separate the Mie and Rayleigh lines in a simple manner.
The invention is particularly useful in aeronautical equipment. Aircraft are sensitive to atmospheric turbulence but it is difficult, at the present time, to anticipate the entry of an aircraft into a region of turbulence, which may generate large mechanical stresses on the structure of the aircraft and/or may injure, quite seriously, passengers and crew members not attached thereto.
Atmospheric turbulence can be detected, at the front of the aircraft, by means of a lidar, by measuring the Rayleigh line in order to detect density and temperature variations in the air. However, the presence of aerosols, and in particular the fluctuation in the density of the aerosols, disturbs the measurement because of a Mie line of variable intensity that is superimposed on the Rayleigh line.